Quasidiabatic states described by coupled-cluster theory

Area of Science:

• Theoretical Chemistry
• Quantum Chemistry
• Computational Chemistry

Background:

• The Koppel, Domcke, and Cederbaum (KDC) model Hamiltonian technique is a valuable tool for studying complex molecular systems.
• Equation-of-motion coupled-cluster (EOM-CC) theory provides accurate descriptions of excited states and electronic properties.
• Jahn-Teller and pseudo-Jahn-Teller interactions significantly influence the behavior of molecular radicals.

Purpose of the Study:

• To extend the utility of the KDC model Hamiltonian technique.
• To develop an ansatz for quasidiabatic wave functions within EOM-CC theory.
• To enable accurate calculations of molecular properties influenced by Jahn-Teller effects.

Main Methods:

• Introduction of an ansatz for quasidiabatic wave functions in EOM-CC theory.
• Development of analytic first derivative theory for the quasidiabatic potential matrix.
• Extension of analytic gradient theory for EOM-CC energy.
• Implementation within the EOM-CCSD (singles and doubles approximation) framework.

Main Results:

• A novel theoretical framework for quasidiabatic wave functions in EOM-CC theory.
• Analytic derivative calculations for quasidiabatic potentials are now feasible.
• Successful implementation for EOM-CCSD calculations on radicals.
• Demonstrated applicability to systems with pseudo-Jahn-Teller and Jahn-Teller interactions.

Conclusions:

• The developed method expands the capabilities of the KDC technique.
• The analytic derivative implementation facilitates the construction of KDC quasidiabatic model potentials.
• This approach offers a robust way to study Jahn-Teller phenomena in molecular radicals using high-level electronic structure theory.